Does an increase in intraluminal pressure of cerebral blood vessels lower the blood-brain barrier to protein? The arterial blood pressure of rats was recorded continuously from the femoral artery through a pressure gauge. After the vasopressor, Aramine, was injected intravenously, the pressure rose from 90 to 150 mm Hg. At this threshold, blood-borne peroxidase escaped from some blood vessels and appeared as brown spots in fixed, incubated sections. The entire brain was sectioned and the brown spots tallied. There was a gradient of exudate numbers, the highest being in the forebrain, the lowest in the brain stem. Further, the total numbers of openings in immature rats was less than in mature animals. When Evans blue dye was given directly before Aramine and HRP sometime later, only blue spots appeared 1 to 2 hours after Aramine infusion; the barrier opening was reversible. When the intraluminal pressure was raised by rapid infusion of a bolus of saline or aldehyde, into one carotid artery, many brown exudates appeared on the injected side within a few minutes. The results indicate that the vascular infusion of a bolus of fluid, including radiocontrast material, may result in a transient break in the blood-brain barrier to proteins.